This invention relates generally to the field of sensor protection including protective eyewear. More particularly, this invention relates to a new and improved protective filter which prevents lasers from damaging the eyes and other sensors.
The increased use of tunable and chirped lasers as range finders, target designators and directed energy weapons (DEW) in the battlefield creates a serious threat to equipment sensors and soldier's eyes. To provide effective protection to such sensors (eyes), a device must have a broad rejection band (or a tunable, narrow band) in addition to fast response, good optical quality and high transmittance in the off-state.
Harmful laser radiation can be prevented from reaching sensors including the human eyes by a number of approaches. If the wavelengths of lasers are known in advance, it is usually possible to use absorptive dyes or fixed wavelength narrow notch reflectors to protect the sensors. Both absorptive glasses and Plastics with synthetic dyes may be used to selectively absorb harmful laser radiation. This approach is particularly attractive if the wavelengths of the lasers are outside the spectral range at which the sensor is designed to operate. However, if the wavelengths of the lasers are inside the spectral range of the sensor, the preferred approach is to use narrow notch reflectors at the required wavelength(s) in the form of dielectric thin films, or rugate filters or volume holographic notch reflectors because these filters provide greater transmittance over the spectral sensitive region of the sensor than absorptive dyes.
However, if the wavelengths of the lasers are not known in advance or the wavelengths of the lasers can be varied with time such as in the case of tunable lasers and chirped lasers, the above approaches will not provide effective protection to the sensor. New approaches that can block frequency agile lasers are required for effective sensor protection. Within the spectral sensitivity range of the sensor, the ideal device will only block harmful laser light at any wavelength from any angle with a high efficiency, and transmit background light completely. The device must have good optical quality and minimum coloration. For practical purposes, the device must also be compact and light weight. Furthermore, if the device actively switches between the blocking and the non-blocking states, the response should be sufficiently fast such that pulsed lasers can also be blocked.
A number of approaches has been suggested in the past. These include: nonlinear scattering, self-focusing, PLZT window, nonlinear absorption, coherence filter, sacrificial filter and a tunable narrow notch reflector. The last approach may be constructed using the approach suggested in U.S. Pat. No. 4,786,128. None of them appears to be ideal for the required application.